Yeah,, their is something going-on in that left side. If the pressure was high enough to blow-out 3 left side calipers, then why would it not blow-out the right side?? Just thinking,,.
In the disc/drum brakes their was a residual valve, which kept about 10lbs pressure in the rear wheel cylinders to prevent the cylinders from trying to suck air when the brakes were released,, that 10lbs kept the wheel cylinder cups expanded.
In a disc/disc system their is no need for a residual valve unless the master cylinder is below the level of the calipers,, and then they use a 2lb valve.
Actually, in the way the duel circuit master works, the pistons are not mechanically connected together, the front brake piston cannot exert much force to the discs until the rear piston has moved enough fluid to push the wheel cylinders out to start applying pressure to the rear drums.
With a disc/disc set-up you should have a disc/disc combination valve, (that's the proper name,) and let the ABS do the work of modulating the pressures in the lines.